Cavity termination for microwave oscillators



y 3, 1966 c. A. BEATY 3,249,890

CAVITY TERMINATION FOR MICROWAVE OSCILLATORS Filed March 27, 196-3 INVENTOR.

CHARLES A. BEATY ATTORNEYS United States Patent 3,249,890 CAVITY TERMINATION FOR MICROWAVE OSCILLATORS Charles A. Beaty, Tampa, Fla. Filed Mar. 27, 1963, Ser. No. 271,042 4 Claims. (Cl. 33198) This invention relates to a microwave oscillator. More specifically it relates to a microwave triode oscillator of the reentrant type having a low impedance termination and having also provision for temperature compensation.

Oscillators of the type here concerned are used in radar and communications applications to provide continuous or pulsed high frequency electrical signals. Usually they employ a coaxial transmission line or cavity having a vacuum tube positioned within the cavity and electrically connected at its plate and cathode to the inner and outer conductors respectively of the line for supplying energy thereto. These devices also have a conducting grid member or sleeve electrically connected to the grid of the vacuum tube and extending within the cavity, coaxially with the inner and outer conductors thereof. The grid sleeve is the means by which electrical energy from the cavity is coupled back to the tube grid in proper phase and amplitude for oscillatory signal regeneration.

A relatively high direct biasing voltage must be applied between the plate and cathode of the tube, or in other words between the inner and outer conductors of the oscillator, and for this the inner or plate line conductor is extended exteriorly of the cavity and electrically connected to a positive voltage supply. The RF. energy generated within the cavity is coupled conveniently to the load through a side access opening in the outer conductor.

One difficulty with microwave oscillators is that the RF. fields within the oscillator cavity tend to follow the inner conductor or plate line to the outside of the oscil lator where they radiate or are otherwise dissipated, thereby disturbing adjacent circuitry and greatly reducing the power available to be coupled to the load. Oscillator efficiency suffers, and in extreme cases the RF. losses become so great that oscillations can no longer be sustained within the cavity.

Prior microwave oscillators have employed various devices to confine the RF. fields within the cavity and yet permit high direct voltages to be applied between the inner conductor or plate line and the outer conductor. For example, a by-pass capacitor located at the entry of the plate line into the cavity has been utilized for this purpose. More commonly a large diameter quarter-wave choke joint electrically connected to the plate line and slidably mounted within the cavity itself has been used.

While these prior devices adequately performed their isolating or choking function, they have been found to considerably increase the capacitance between the inner and outer conductors. In the case of the by-pass capacitor, the large capacity is required to properly isolate the R.F. and direct voltage circuits; while in the case of the choke joint, the large capacitance is due to the large surface area of the joint itself. In any event, this capacitance causes a considerable distortion of the incoming video pulses when the device is operated in the plate pulsed mode.

Moreover, oscillator devices utilizing the choke joints must rely on plate line tuning. There is no room to insert tuning means such as a trimmer capacitor into the cavity. Accordingly, then, tuning range is limited by the small distance through which the choke joint can slide within the cavity. Microwave oscillators heretofore have suffered also because they are unable to accommodate themselves -to large changes in temperature. When used for example as a local oscillator in transmitting circuits,

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they succeed in matching the frequency drift of the magnetron only over a relatively short temperature range.

Accordingly, a principal object of this invention is to provide an improved triode microwave oscillator, capable of CW. or pulsed operation, in which the RP. fields are confined entirely within the oscillator cavity.

Another object of the invention is to provide an oscillator which is accurately tunable over a relatively wide range of frequencies as compared with prior comparable devices and which is temperature compensated over a wide range of temperatures.

A further object is to provide a microwave oscillator which is small, but easy to assemble; inexpensive to manufacture, but rugged, and which suffers substantially no power losses due to unwanted R.F. radiation.

It is a more specific object of this invention to provide an oscillator capable of plate pulsed operation which neither distorts nor otherwise adversely affects the incoming video pulses.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and object of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawing, in which:

FIG. 1 is a vertical section of a microwave oscillator embodying my invention, and with portions shown in side elevation;

FIG. 2 is a view along line 2--2 of FIG. 1;

FIG. 3 is a view along line 33 of FIG. 1; and

FIG. 4 is an exploded perspective view of selected portions of the oscillator of FIG. 1.

Referring first to FIG. 1, the oscillator comprises an elongated, generally cylindrical, tubular outer conductor or shell 10. Shell 10 is formed with a stepped, reduced diameter portion or shoulder 11 set in from the left end thereof for positioning a conventional high frequency triode vacuum tube 12. Tube 12 has the usual heater pins 13, 13, cathode ring 14, grid ring 15 and plate pin 16. When the tube 12 is mounted properly within shell 10, the cathode ring 14 seats in the stepped portion of shoulder 11 thereby locating the tube centrally within the shell and at the same time electrically connecting the cathode ring 14 thereto. A conducting retaining ring 17 is fitted within the end of shell 10 followed by an annular insulating spacer member 18. Filament voltage is applied to the heater pins 13, 13 by means of socket connectors 19, 19 which engage pins 13, 13 and extend on out to the end of shell 10. An insulating disc-like member 21 having suitable pair of openings 22, 22 accommodating the connectors 19, 19 is fitted into the end of shell 10. The entire aforementioned tube assembly is held firmly in place by means of a snap ring 23 which =fits tightly in a circumferential groove provided just inside the end of shell 10.

A tubular grid member or sleeve 24, operating in the half wave mode is spaced coaxially within the outer conductor 10. Sleeve 24 extends part way along the length of conductor 10 and its diameter is slightly larger than that of tube 12. The end portion of sleeve 24 facing tube 12 is slotted lengthwise to form an annular array of spring fingers 25 (FIGS. 1 and 2) which are adapted to engage over and make good electrical contact with grid contact ring 15. The individual fingers 25 are grooved at 27 to maintain secure engagement between sleeve 24 and ring 15. The grid sleeve 24 also is supported by a set of adjustable insulating set screws 28 screwed through threaded openings disposed evenly about the circumference of shell and bearing on the outside of the sleeve. Proper adjustment of screws 28 maintains sleeve 24 in coaxial alignment within conductor lit. A resistor 29 is shown connected between the grid-sleeve 24 and cathode ring 14 to provide a DC). return to the grid, although if desired this resistor may actually be built into tube 12.

The oscillator has also an inner conductor or plate line indicated generally at 31 spaced coaxially within shell 10 and also Within grid sleeve 24. In the illustrated embodiment the line 31 operates in the three-quarter wave length mode. Plate line 31 comprises an elongated inner line section 32 extending a considerable distance beyond grid member 24. Section 32 has a diameter slightly larger than that of plate pin 16 and is partially slotted lengthwise from the end facing tube 12 so as to form a pair of curved fingers 33 tightly engaging the pin 16. Line section 32 is seen also to be hollowed out at its other end and is internally threaded at 34 to accommodate the correspondingly threaded end 35 of an elongated outer plate line section 36. The other end of line section 36 is flanged at 37 and extends on out to the end of shell 10 where it can be suitably connected electrically at 38 to a constant or pulsed high voltage supply.

In accordance with this invention, means are provided for terminating in a short circuit the coaxial line or cavity formed between the inner and outer conductors 31 and 10 respectively and thereby confining the R.F. fields within the oscillator cavity, without short circuiting the high direct voltage between the plate line 31 and outer conductor 10. The terminating means are shown in the illustrated embodiment to comprise a tubular conducting member 39 (FIGS. 1 and 4). Member 39 is secured within and so as to make good contact with an enlarged bore portion 41 of shell 10. It is actually made up of two integral portions. The first portion is a thin walled generally cylindrical sleeve portion 42 whose inside diameter is somewhat larger than that of plate line 31 and which is adapted to be positioned coaxially and in closely spaced relation about plate line 31 beyond the free end of grid member 24. The second portion is a circular flange portion 43 arranged in coaxial alignment with the butt end of sleeve 42 facing the open end of shell 10.

An axial opening 44 is formed in disk 43 centrally of sleeve 42 for receiving the plate line 31 in its passage out to the end of shell 10. The diameter of opening 44 is made smaller than that of the bore of sleeve portion 42, thereby forming an annular internal shoulder at 45, .the reason being that as mentioned previously, only the outer reduced diameter section 36 of plate line 31 extends to the end of shell 10. The inner line section 32 is seen .to terminate inside the shell 10 within sleeve portion 42, forming within the sleeve an annular shoulder 32a spaced close to and oflset from shoulder 45. Reducing the diameter of the opening through conducting member 39, then, com pensates for the reduced diameter of the plate line 31 and preserves substantially the same spacing between the plate line and the sleeve and disk portions of member 39.

Thus, when plate line 31 is positioned properly with respect to the conducting member 39, it forms therewith a small diameter, open ended, coaxial transmission line extending from the free end 46 of sleeve 42 to a point 47 at the outer face of flange portion 43.

In accordance with .the invention, the overall length of conducting member 39 is selected so that the transmission line 4647 operates in the quarter wave length mode at the operating frequency of the oscillator. Due to the spacing between the inner and outer conductors 31, 10, respectively of the oscillator, a substantially infinite impedance appears at point 47 and this is reflected as a nearly zero input impedance at point 46, thereby providing a low impedance R.F. circuit termination at point 46 without also providing a DC. short circuit between the plate line 31 and conducting member 39. Thus, the transmission line 46-47 serves as a quarter wave length choke section tot confining the R.F. fields within the oscillator cavity while still permitting high direct voltage to be applied to the plate line.

In order for the transmission line 46-47 to function most efliciently as a choke, its characteristic impedance should be made as small as possible. Accordingly the spacing between conducting member 39 and plate line 31 should be made small. To insure that these two conduc tors remain insulated from one another when the customary high plate voltgae is applied between plate line 31 and outer shell 10, an insulating sleeve 48 is inserted into the space between conductor 39 and plate line 31. The sleeve 48 comprises, in the illustrated embodiment, a generally cylindrical tube constructed of a suitable relatively rigid, low-loss dielectric material such as, for example, polytetrafluoroethylene. Sleeve 48 is formed with a major portion 49 fitting snugly within sleeve 42 and around plate line 31; it also has a reduced diameter minor portion 51 fitting snugly within the opening 44 in disk portion 43 of the member 39. The length of minor portion 51 is approximately equal to the thickness of disk portion 43, while the major spacer portion 49 is made longer than and to extend appreciably beyond the left end of sleeve 42 to prevent arcing between sleeve portion 42 and plate line 31.

Still referring to FIG. 1, spacer portion 49 is provided with an internal bore 52 accommodating the inner plate line conductor 18 and a reduced diameter bore 54 extending through disk portion 43. Bore 54 is internally threaded at 55 to engage a correspondingly threaded shank portion 56 of plate line section 36. A washer 57 constructed of a suitable dielectric material is provided on outer plate line section 36 adjacent to a flange 37 to insulate that from the disk portion 43 of conducting member 39.

With reference first to FIG. 4, in assembling the oscillator, the insulating washer 57 is first slid onto the outer plate line section 36 to abut the flange 37, after which the section 36 is inserted through opening 44 in conducting member 39 until washer 57 engages member 39. Then the insulating sleeve 48 is passed over the end of line section 36 protruding from sleeve 42 and is slid into member 39 until the end of major spacer portion 49 engages shoulder 45 (FIG. 1). Next, the sleeve 48 is screwed onto the threaded shank portion 56 of line 36, thereby locking all of the aforementioned elements together in a single unit. Next the inner plate line section 32 is inserted into the space between sleeve 42 and the outer line section 36 and is screwed onto the latter member until the end 32a engages the inside shoulder formed by the reduced diameter bore portion 54 of spacer 48. Finally, the free end of the inner plate line section 32 is inserted into the oscillator cavity so that its spring fingers 33 engage plate pin 16, and the entire end plug assembly is then seated in the end portion 41 of shell 10 and secured therein by screws or other convenient means (not shown).

When properly seated within shell 10, the conducting member 39 etfectively seals the oscillator cavity against dirt and moisture. Further, the disk portion 43 of member 39 is spaced approximately one quarter wave length from the free end of grid member 24 so that the plate line 31 can operate as desired in the A wave length mode. Thus energy is coupled back into the grid-cathode line in phase for sustaining oscillations within the cavity. Energy may be coupled out of the cavity through a conventional probe, indicated at 58, mounted through the side Wall of shell 10.

It is a feature of this invention that the open ended quarter wave transmission line 46-47 forming the RF. terminating choke section is located substantially at the axis of the oscillator and thus a substantial distance from the shell 10. Accordingly the portion of the oscillator beyond the grid member is free to receive a tuning stub indicated generally at 59 mounted through the side Wall of shell 10. The illustrated tuning stub, which efiects capacitance tuning, comprises a threaded screw 61 extending through a correspondingly threaded and partially slitted sleeve 62 integral with shell 10. The outside of sleeve 62 is also threaded and is adapted to receive a correspondingly threaded nut 63 which when screwed down on sleeve 62 squeezes it together and thereby locks the tuning screw 61 in place.

As best seen in FIG. 1, the inner end of the tuning screw 61 is free to travel all the way from shell to sleeve 42 or in other words, almost over the entire radius of the oscillator cavity. Accordingly, the oscillator is accurately tunable over a relatively wide range of frequencies as compared with prior comparable devices. Further, the tuning screw 61 can be screwed all the way down so as to touch sleeve 42 without shorting out the high voltage D.C. plate circuit, since both the screw 61 and the sleeve 42 are always maintained at the same direct electrical potential as the outer shell 10.

A further advantage of the centrally located transmission line 46-47 is that a large space is provided within the cavity for accommodating temperature responsive means to compensate the oscillator for the effects of temperature change. The temperature responsive means shown in the illustrated embodiment (FIG. 1) comprise three generally L-shaped bimetallic strips 64 disposed radially about the axis of shell 10. The strips 64 are secured at their short legs by rivets 65 to the inside face of disk portion 43. The longer legs of the strips on the other hand are free to swing inwardly toward the sleeve portion 42 when heated, thereby decreasing the capacitance of the anode line and tending to increase the frequency of the oscillator. As seen in FIG. 1, the ends of strips 64 are free to swing all the way from outer conductor 10 to sleeve 42. Resultantly they can be made to have extremely sensitive response over a relatively wide range of temperatures. The strips may be arranged to compensate for frequency drift of the oscillator caused by temperature changes and thus stabilize the oscillator frequency. On the other hand, they may be arranged to match the drift of another device, such as a magnetron, used in conjunction with the oscillator.

It should be noted that the centrally-located choke section 46-47 has substantially no eflect on the waveform of the incoming video pulses when the oscillator is operated in the plate pulse mode. This is because the sleeve 42 forming the outer conductor of the choke section 46-47 is spaced at a considerable distance away from outer conductor (shell 10). Furthermore, its surface area is quite small. The result therefore is that very little capacitance is added to the plate line to eflect the arrive time of the incoming pulses at the plate line.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efliciently attained and, since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention, which, as a matter of langauge, might be said to fall therebetween.

I claim:

1. A microwave oscillator for developing electrical signals of selected frequencies, said oscillator comprising, in combination:

(A) a tubular outer conductor;

(B) a coaxial inner conductor;

(C) a vacuum tube mounted coaxially within one end of said outer conductor, said tube having (1) a cathode electrode electrically connected to said outer conductor,

(2) an anode electrode electrically connected to said inner conductor and receiving D.C. energization through said inner conductor, and

(3) a grid electrode;

(D) a grid member disposed coaxially between said inner and outer conductors, said grid member (1) being electrically connected to said grid electrode,

(2) cooperating with said inner conductor to define a grid-plate cavity,

(3) cooperating with said outer conductor to define a grid-cathode cavity, and

(4) having a free end beyond which said inner and outer conductors cooperate to define a platecathode cavity;

(E) rneans terminating said plate-cathode cavity, said terminating means including (1) an electrically conducting sleeve disposed coaxially about said inner conductor and in capacitively coupled relationship thereto, and

(2) a radial flange fixedly mounted within the other end of said outer conductor, said flange (a) being located adjacent the end of said sleeve remote from said tube and physically contacting both said sleeve and said outer conductor to physically terminate said platecathode cavity, and seal off the interior of said outer conductor from the external environment,

(b) electrically contacting said sleeve and said outer conductor to electrically terminate said plate-cathode cavity, and

(c) cooperating with said sleeve and said inner conductor to provide a low impedance, open ended coaxial transmission line operating in the quarter wave length mode so as to substantially prevent leakage of radio frequency energy from said plate-cathode caviity past the electrical termination thereof,

(3) the location nearest said tube of the physical and electrical contact between said terminating means and said outer conductor being radially aligned with the radial surface of said flange nearest said tube; and

(F) a tuning screw adjustably projecting through said outer conductor into said plate-cathode cavity radially toward said sleeve for adjusting the selected signal frequency developed by said oscillator.

2. A microwave oscillator as defined in claim 1 and a plurality of bi-metallic strips secured within said platecathode cavity between said sleeve and said outer conductor.

3. The oscillator defined in claim 1 and a dielectric sleeve disposed in said transmission line to insulate said electrically conducting sleeve and said flange from said inner conductor.

4. A microwave oscillator for developing electrical signals of selected frequencies, said oscillator comprising, in combination:

(A) a tubular outer conductor;

(B) a plate line disposed coaxially within said outer conductor;

(C) a vacuum tube mounted coaxially within one end of said outer conductor, said tube having (1) a cathode electrode electrically connected to said outer conductor,

(2) an anode electrode electrically connected to said plate line and receiving D.C. energization therethrough, and

(3) a grid electrode;

(D) a grid member disposed coaxially between said plate line and said outer conductor, said grid member (1) being electrically connected to said grid electrode,

(2) cooperating with said plate line to define a grid-plate cavity,

(3) cooperating with said outer conductor to define a grid-cathode cavity, and

(4) having a free end beyond which said plate line and said outer conductor cooperate to define a plate-cathode cavity;

(E) means terminating said plate-cathode cavity, said terminating means including (1) an electrically conducting sleeve disposed coaxially about said plate line in closely spaced insulated relationship thereto,

(a) the end of said conducting sleeve extending somewhat axially beyond the end of said plate line,

(2) a radial flange fixedly mounted within the other end of said outer conductor to physically terminate said plate-cathode cavity and seal off the interior of said outer conductor from the external environment,

(a) said flange abutting said end of said conducting sleeve to electrically connect it to said outer conductor,

(b) said flange having a central bore of a diameter somewhat less than the inside diameter of said conducting sleeve,

(3) a coaxial plate line extension connected to said inner conductor and extending through said central bore in closely spaced relationship to said flange for connection to an external D.C. supply source,

(4) a dielectric sleeve insulating said conducting 8 sleeve from said plate line, said conducting sleeve from said extension, and said flange from said extension,

(5) the combination of said conducting sleeve and said flange cooperating with the combination of said plate line and said extension to define a quarter wave open ended transmission line of uniformly low characteristic impedance for electrically terminating said plate-cathode cavity, and thereby preventing leakage of RF energy from said plate-cathode cavity;

(F) a tuning screw adjustably projecting through said outer conductor into said plate-cathode cavity toward said conducting sleeve so as to adjust the selected signal frequency developed by said oscillator; and

(G) plural temperature responsive bi-metallic strips secured to said flange and operating in said plate-cathode cavity between said conducting sleeve and said outer conductor.

References Cited by the Examiner UNITED STATES PATENTS 2,429,811 10/ 1947 Guarrera 331-98 2,451,825 10/1948 Guarrera 331-98 2,561,727 7/1951 Cooper et a1. 333-82 X 2,994,042 7/1961 Power et a1. 331-98 ROY LAKE, Primary Examiner.

JOHN KOMINSKI, J. B. MULLINS,

Assistant Examiners. 

1. A MICROWAVE OSCILLATOR FOR DEVELOPING ELECTRICAL SIGNALS OF SELECTED FREQUENCIES, SAID OSCILLATOR COMPRISING, IN COMBINATION: (A) A TUBULAR OUTER CONDUCTOR; (B) A COAXIAL INNER CONDUCTOR; (C) A VACUUM TUBE MOUNTED COAXIALLY WITHIN ONE END OF SAID OUTER CONDUCTOR, SAID TUBE HAVING (1) A CATHODE ELECTRODE ELECTRICALLY CONNECTED SAID OUTER CONDUCTOR, (2) AN ANODE ELECTRODE ELECTRICALLY CONNECTED TO SAID INNER CONDUCTOR AND RECEIVING D.C. ENERGIZATION THROUGH SAID INNER CONDUCTOR, AND (3) A GRID ELECTRODE; (D) A GRID MEMBER DISPOSED COAXIALLY BETWEEN SAID INNER AND OUTER CONDUCTORS, SAID GRID MEMBER (1) BEING ELECTRICALLY CONNECTED TO SAID GRID ELECTRODE, (2) COOPERATING WITH SAID INNER CONDUCTOR TO DEFINE A GRID-PLATE CAVITY, (3) COOPERATING WITH SAID OUTER CONDUCTOR TO DEFINE A GRID-CATHODE CAVITY, AND (4) HAVING A FREE END BEYOND WHICH SAID INNER AND OUTER CONDUCTORS COOPERATE TO DEFINE A PLATECATHODE CAVITY, (E) MEANS TERMINATINGA SAID PLATE-CATHODE CAVITY, SAID TERMINATING MEANS INCLUDING (1) AN ELECTRICALLY CONDUCTING SLEEVE DISPOSED COAXIALLY ABOUT AN INNER CONDUCTOR AND IN CAPACITIVELY COUPLED RELATIONSHIP THERETO, AND (2) A RADIAL FLANGE FIXEDLY MOUNTED WITHIN THE OTHER END OF SAID OUTER CONDUCTOR, SAID FLANGE (A) BEING LOCATED ADJACENT THE END OF SAID SLEEVE REMOTE FROM SAID TUBE AND PHYSICALLY CONTACTING BOTH SAID SLEEVE AND SAID OUTER CONDUCTOR TO PHYSICALLY TERMINATE SAID PLATE CATHODE CAVITY, AND SEAL OFF THE INTERIOR OF AND OUTER CONDUCTOR FROM THE EXTERNAL ENVIRONMENT, (B) ELECTRICALLY CONTACTING SAID SLEEVE AND SAID OUTER CONDUCTOR TO ELECTRICALLY TERMINATE SAID PLATE-CATHODE CAVITY, AND (C) COOPERATING WITH SAID SLEEVE AND SAID INNER CONDUCTOR TO PROVIDE A LOW IMPEDANCE, OPEN ENDED COAXIAL TRANSMISSION LINE OPERATING IN THE QUARTER WAVE LENGTH MODE SO AS TO SUBSTANTIALLY PREVENT LEAKAGE OF RADIO FREQUENCY ENERGY FROM SAID PLATE-CATHODE CAVIITY PAST THE ELECTRICAL TERMINATION THEREOF, (3) THE LOCATION NEAREST SAID TUBE OF THE PHYSICAL AND ELECTRICAL CONTACT BETWEEN SAID TERMINATING MEANS AND SAID OUTER CONDUCTOR BEING RADIALLY ALIGNED WITH THE RADIAL SURFACE OF SAID FLANGE NEAREST SAID TUBE; AND (F) A TUNING SCREW ADJUSTABLY PROJECTING THROUGH SAID OUTER CONDUCTOR INTO SAID PLATE-CATHODE CAVITY RADIALLY TOWARD SAID SLEEVE FOR ADJUSTING THE SELECTED SIGNAL FREQUENCY DEVELOPED BY SAID OSCILLATOR. 